Murine sarcoma virus-transformed mouse fibroblasts produce polypeptide growth factors and release them into serum-free medium. These factors stimulate cells to divide in monolayer cultures and also to form colonies that grow progressively soft agar. Three major peaks of activity are seen, with apparent molecular weights of 25,000, 12,000, and 7000. The sarcoma growth factors are heat-stable, trypsin-sensitive, and active in nanogram quantities when tested for growth stimulation of untransformed rat and mouse fibroblasts. All three molecular species are also capable of competing for membrane epidermal growth factor (EGF) receptors when tested with 125I-labeled EGF. They differ from mouse EGF, however, in their molecular weights, in their inability to react with anti-EGF antibodies, and in their ability to convert cells to anchorage independent (agar) growth. For the above reasons, we conclude that the sarcoma growth factors are a new class of polypeptide tropic factors that confer on fibroblasts in vitro properties associated with the transformed phenotype.